Incorrect type inference of Promise.all when spreading Promise arrays
  • 11-Jun-2023
Lightrun Team
Author Lightrun Team
Incorrect type inference of Promise.all when spreading Promise arrays

Incorrect type inference of Promise.all when spreading Promise arrays

Lightrun Team
Lightrun Team

Explanation of the problem

The problem arises in TypeScript version 4.0.2, including the TypeScript Playground, when using Promise.all with mixed types. The expected behavior is that Promise.all should not throw any compile-time errors due to type mismatches. As per the documentation, Promise.all returns the values of the array and waits for their resolution if any value is a promise. However, the actual behavior is that a compile-time error is thrown, indicating a type mismatch.

In the provided code snippet, the function test is declared as an async function. Within the function, two promises are created: promiseNumber which resolves to the number 1, and promiseVoid which is an async function returning void. The problematic line is where Promise.all is called with an array containing promiseNumber and the spread syntax for promiseVoid(). This combination triggers a compile-time error because TypeScript cannot infer the correct types when dealing with mixed promises and non-promise values.

A workaround is available to resolve this issue by explicitly setting the generic type argument of Promise.all to number | void. This informs TypeScript that the first argument returned from the Promise.all call will be a number. By specifying the type explicitly, the compile-time error is avoided. However, it is worth noting that this workaround is a step backward in terms of TypeScript’s improved Promise.all inference capabilities and should not be necessary given the expected behavior of Promise.all. The provided workaround should be considered until the underlying issue is resolved.


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Problem solution for Incorrect type inference of Promise.all when spreading Promise arrays

  1. The provided answers indicate a general consensus that the issue mentioned in the question has been resolved. To delve deeper into the technical aspect of the problem, the issue was related to type inference in TypeScript’s Promise.all function. In the code example, the function test() uses Promise.all with an array containing a Promise<number> and the spread of a Promise<void>. The expected behavior is that Promise.all should correctly infer the types and not throw any compile-time errors. However, prior to the fix, TypeScript would generate an error stating that the types returned by [Symbol.iterator]().next(...) were incompatible.

    The proposed solution involved setting the generic type argument of Promise.all to number | void, which acted as a workaround to bypass the error. However, this was seen as a step backwards since TypeScript had already introduced improved type inference for Promise.all. Developers were expecting TypeScript to infer the correct types without explicitly specifying them.

    Thankfully, the issue has been resolved in recent TypeScript versions. The fixes implemented in TypeScript, such as version 4.0.2 and onwards, addressed the problem with type inference in Promise.all. As a result, developers can now rely on TypeScript’s enhanced type inference capabilities to accurately determine the types of the resolved promises within Promise.all. This means that developers no longer need to use the workaround of manually specifying the generic type argument as number | void to avoid compilation errors. With the fix in place, the code can now be written as originally intended, without any explicit type annotations, and TypeScript will correctly infer the types based on the promises provided to Promise.all.

    In conclusion, the issue of type inference in Promise.all has been resolved in recent TypeScript versions. The fix ensures that TypeScript accurately infers the types of resolved promises within Promise.all, eliminating the need for manual type annotations or workarounds. Developers can now rely on TypeScript’s improved type inference capabilities to handle promises within Promise.all more effectively and without encountering compilation errors. This enhancement simplifies the code and allows developers to write more concise and maintainable TypeScript applications.


Other popular problems with Microsoft TypeScript


Problem: Incorrect Use of TypeScript Interfaces

TypeScript interfaces are a powerful tool for enforcing strict type checking in a codebase. However, incorrect use of interfaces can lead to problems with code accuracy and maintainability. For example, if an interface is defined with properties that are not used elsewhere in the code, it can be difficult to track down the source of an error later on.


To avoid this problem, it is recommended to make use of strict null checking and optional properties in interfaces. Additionally, be mindful of the properties and methods defined in an interface, and make sure that they are actually used elsewhere in the code. If an interface is no longer needed, it should be removed to prevent confusion and errors.

Problem: TypeScript Compilation Errors

TypeScript is a statically-typed language, which means that all type information is known at compile time. This can lead to compilation errors when code is written that violates TypeScript’s type system. For example, if a variable is declared with a type of string, and an attempt is made to assign a value of type number to it, a compile-time error will occur.


To resolve TypeScript compilation errors, it is important to carefully review the code and make sure that all variables are correctly declared with the correct type. In cases where a variable needs to be used with different types, a union type can be used to specify multiple types for the same variable. Additionally, the TypeScript documentation provides detailed information about the type system, and can be a valuable resource for resolving compilation errors.

Problem: Managing TypeScript Dependencies

Managing dependencies in a TypeScript project can be challenging, as different libraries and packages may have different versions and compatibility requirements. This can lead to conflicts and errors when attempting to use multiple libraries that have incompatible dependencies.


To resolve dependency management issues in a TypeScript project, it is recommended to make use of a package manager such as npm or yarn. These tools provide automated dependency management, and can help to prevent conflicts and errors when using multiple libraries and packages. Additionally, it is important to keep dependencies up-to-date, as newer versions may resolve compatibility issues and improve the overall stability of the project.

A brief introduction to Microsoft TypeScript

Microsoft TypeScript is a statically-typed, open-source programming language that builds on JavaScript. It is designed to provide optional type safety, improved tooling, and enhanced scalability to JavaScript code. TypeScript offers a language structure that is familiar to JavaScript developers, but with the added benefits of static type checking and enhanced tooling support.

TypeScript is designed to be compatible with existing JavaScript code and integrates seamlessly into many popular development environments and build tools. The language offers features such as class and interface definitions, type inference, and advanced type checking, making it easier for developers to write robust, maintainable code. TypeScript also includes a transpiler that can convert TypeScript code into equivalent JavaScript code, allowing developers to write TypeScript code that can run in any environment that supports JavaScript.

Most popular use cases for Microsoft TypeScript

  1. Large-scale web application development: TypeScript is well-suited for developing large-scale web applications, as it provides developers with the ability to write scalable, maintainable code. With its optional type checking, developers can catch type-related errors at compile time, making it easier to catch bugs and reduce the time spent debugging code. Additionally, TypeScript’s compatibility with existing JavaScript code allows developers to gradually adopt the language in their existing codebases, making it easier to transition to a statically-typed codebase.
class User {
    name: string;
    email: string;

    constructor(name: string, email: string) { = name; = email;

const user = new User("John Doe", "");
  1. Improved tooling support: TypeScript integrates well with modern development environments and build tools, making it easier for developers to write, manage, and maintain code. With TypeScript’s enhanced tooling support, developers can benefit from features such as code completion, refactoring, and debugging, which can help to increase developer productivity and reduce the time spent on manual code management tasks.
  2. Interoperability with JavaScript libraries: TypeScript is designed to be compatible with existing JavaScript code, making it easy for developers to integrate TypeScript with existing JavaScript libraries and codebases. Additionally, TypeScript provides a way to define type information for JavaScript libraries, making it easier to write TypeScript code that interacts with existing JavaScript libraries in a type-safe manner. This can help to reduce the time spent debugging and improve the overall stability of code.

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